The antineoplastic agent, 4(5)-(3,3-dimethyl-1-triazeno)imidazole- 5(4)-carboxamide (DTIC, NSC 45388) and other aryldialkyltriazenes have been shown to be carcinogens in the rat. DTIC is the only one of the compounds which has induced thymic lymphosarcomas and leukemias in the rat. DTIC and the aryldialkyltriazenes were shown to undergo N- demethylation in vivo and in vitro. The resulting aryl- monoalkyltriazenes were suggested as proximate carcinogens since they are known alkylating agents. An alternative mechanism of carcinogenesis by DTIC could be activation of a latent oncogenic virus. At autopsy, rats fed DTIC grossly resemble rats infected with Gross leukemia virus. Studies will be carried out so as to confirm or deny the presence of a viral agent. If a virus is present, its importance in the genesis of thymic lymphosarcomas will be studied. This work involves a pathogenetic study of the thymus of rats fed DTIC. Both light and electron microscopy will be used. Cell free transfer experiments will be performed. A group of animals will also be administered DTIC and its known metabolites so as to aid in identification of a possible proximate carcinogen.